Electrical resistor



Dec. 9, 1958 A. BOL ETAL 2,863,340

ELECTRICAL RESISTOR Filed Jan. 25, 1957 2 Sheets-Sheet 1 FIGJ ARIE BOLCHRISTIAAN ALFONS MARIA KLAASSEN PIETER JOHANNES WILHELMUS JOCHEMS Dec.9, 1958 A. BOL ETAL 2,863,340

ELECTRICAL RESISTOR Filed Jan. 25, 1957 2 Sheets-Sheet 2 ELECTRICALRESISTOR Arie Bol, Christiaan AlfonsMaria Klaassen, and Pieter JohannesWilhelmus .lochems, .Eindhoven, Netherlands, assignors, by inesneassignments, to North American Philips Company, Inc., New York, N. .Y..,a corporation of Delaware Application Sanitary 25, 1957, Serial No.636,316

Claims priority, application Netherlands January 31, 1956 4 Claims. (Cl.252-519) Our invention relates to negative temperature coefficientresistor and to method of producingthe same.

U. S. patent application Ser. No. 278,494, filed March 25, 1952, nowabandoned, discloses negative temperature coetficient resistorshaving'high negative temperature coeflicients and-high stabilitieswhichare fo-rmedby. sintering together lithium oxide and nickel oxide inan oxidizing atmosphere. However, the properties of such resistors varygreatly with the source of the nickel oxide or nickel carbonate used.

A principal object therefore of our invention is to provide a means forneutralizing the variation in the prop-, erties of these resistors duetothe variation in the source of the nickel oxideor nickel carbonate.

This and other objects of our invention will. be ap parent from thedescription that follows:

According to our invention we have found that'variations in theproperties of a sintered lithium oxide-nickel oxide negative temperaturecoefiicient resistor maybe greatly reduced by incorporating a quantityof copper ox-v ide in the resistor. More particularly we have found thatthe variations in the properties of nickel oxide resistors having alithium oxide content from 0.1 to mol percent may be greatly reduced byincorporating in these resistors from 1 to 30 mol percent preferablyfrom 10 to mol percent; of copper oxide calculated a CuO. The additionof copper oxide in an amount less than lmol percentis too small toprovide the favorable effects of the in:

vention while the, addition of copper oxide in an amount.

greater than mol percent-is not satisfactory since thesecompositionshave a sintering range that is so .short that the resistorscannotbe easily produced.

The'resistors of our invention are produced by-mixmg oxides of Ni, Liand Cu or compounds which form these oxides upon heating such as lithiumcarbonate andnickel carbonate, moulding the, resultant mixtur with theaid: of a binding agent such as methylacrylate or, starch and sinteringthe resultant moulded, body in an oxidizing at.-.. mosphere, preferablyair, at a temperature of from about.

1100 C. to 1300 C.

Our invention will now be described in greater detail with reference tothe following examples, tables and drawing, Figure 1 of which is a graphshowing theeffect of the addition of a fixed amount of CuO on thespecific resistances at 30 C. on nickel-oxide-lithiurn oxide resistorscontaining varying amounts of lithium oxide and Figure 2 of which is agraph showing the effect ofa variation on the CuO content on thespecific resistances at 30 C. of nickel oxide-lithium oxide resistors.

The composition of nickel oxide varies with the source. Certainimpurities are usually included in commercial source of nickel oxidewhich, it has been found adverse 1y affect the resistance properties ofa resistor prepared in accordance with the disclosure of the U. S.application referred to above as indicated in Table 2.

Table 1 shows for a plurality of such initially nickel oxide materialsthe spectrochemically determined content of impurities in percent byweight.

lCe

TABLE 1 I II III The preparation indicatedby is a nickel carbonate whichhas been converted by. heating into nickel oxide. A portion thereof hasbeen heated at 1200 C. for 5" hour and another portion at 900 C. for 5hours. These products, which have different reactivities, are'indicatedhereinafter-by lVa andlVb, respectively. Furthermore, a portion ofthenickel-oxide preparation V has been made less reactive by heating at 1000 C. for 4 hours.- The last-mentioned nickel oxidewill'be-indicatedhereinafterby Va.

Example; I

Resistors Were manufacturedjromthe different nickel oxidepreparations.cor-responding. to Table-.1, in thefollowing manner. The nickel oxidewas ground inaball; mill with alcohol for; 16. hours. with the; additionof lithium carbonate in. an amount. such that the mixture contained 1.5mol percent of Li O. Theili thium carbonate-was preventedfro-mdissolving in thealcohol by the use of alcohol saturated withlithium. carbonate. After filtration and drying for example 5 gs. of. asolu; tion of 20 gs.;of methyl acrylate witha softening agent of 1 ml.of dibutylphthalate in mls. of methylacetate, was added asv a bindingagent for 100 gs. of powder. Rods of 5-x 5 x. 15 mms. were moulded fromthis mass. and sintered inair at 1,240 C. forlho-ur.

The following table,- Table 2 showsforv resistors ;ob-. tained from aplurality of nickel oxide preparations with an addition of 1.5 molpercent-oflithium oxide the values of the specific resistance at 30" C.(p ),i11 (2 cm. and the values in K. of the factorB in theformula:

PT: 116T wherein A indicates a constant, e indicates the base of thenatural logarithm, T indicates the absolute temperature and in which oindicate the specific resistance at the absolute temperature T. As canbe noted from the formula the factor B is a measure of the temperaturecoefficient of the resistor. I

TABLE 2 Nickel oxide +1.5 mol percent of LizO p30. B

From Table 2 it is clear that there is a considerable variation in theresistance properties of the resistors formed by the method of ExampleI, due to the source of the nickel oxide and/ or the preliminarytreatment given to it. Thus it is noted that the values of p vary from1,365 to 170,000 2 cm. and the value of B varies from 3,330 to 4,370" K.

Example II Resistors were manufactured according to the method ofExample I using the compositions of Table 2 except that part of thenickel oxide was replaced by 15 mol. percent of CuO.

The values of the specific resistance at 30 C. (p in S: cm. and thevalues of the factor B in K. of the resistors of Example II are shown inTable 3 which follows:

TABLE 3 Nickel oxide +1.5 moi. percent of Llz+15 p30 13 mol. percent ofCuO Table 3 shows that the variation in properties between the resistorswhen using the different nickel-oxide preparations, which for a contentof 1.5 mol. percent of M 0 was from 1,365 to 170,000 9 cm. for the p andfrom 3,300 to 4,370" K. for the factor B, has been reduced by a contentof copper oxide according to the invention to values for the p and thefactor B between 100 and 300 9 cm. and 3,250 and 3,965 K., respectively.

The etfect of the addition of copper oxide to lithium oxide-nickel oxideresistors containing varying amounts of lithium oxide is shown in FigureI which is a graph in which the specific resistance p is plotted againstthe mol. percent of Li O of various nickel oxide-lithium oxide rcsistorsin which the same source of nickel oxide is used for all the resistors.In the graph curve A represents the etfect of a variation in the lithiumoxide content on the specific resistance at 30 C., p and curve Brepresents the eflect of a CuO content of 10 mol. percent on thespecific resistance p of resistors of a similar content of lithiumoxide. The values measured for the specific resistance p in S: cm. forseveral of the compositions of these curves, together with theassociated values for B in K. are specified in Table 4 which follows:

TABLE 4 L110 CuO p20 B TABLE 5 Nickel Li1O GuO mo 13 oxide V 1% 31, 0003, 540 Va 1% 1,365 3, 330 VI 1% 5,425 3. 430 V 5 1, 740 3, 430 Va 1% 5020 3, 140 VI 1% 5 560 3, 200 V 1% 15 3, 450 Va 1 15 118 3, 330 VI 1% 15100 3, 250

While we have described our invention in connection with specificembodiments and applications, other modifications thereof will bereadily apparent to those skilled in this art without departing from thespirit and scope of the invention as defined in the appended claims.

What we claim is:

1. A sintered electrical resistor having a negative temperaturecoefficient and consisting essentially of from about 0.1 to 10 molpercent of lithium oxide, about 1 to 30 mol percent of copper oxide,calculated as CuO, and the remainder nickel oxide.

2. A sintered electrical resistor having a negative temperaturecoetficient and consisting essentially of from about 0.1 to 10 molpercent of lithium oxide, about 10 to 20 mol. percent of copper oxide,calculated as CuO, and the remainder nickel oxide.

3. A method of manufacturing a sintered electrical resistor having anegative temperature coefiicicnt comprising forming a mixture of theoxides of Ni, Li and Cu in amounts such that from about 0.1 to 10 mol.percent of lithium oxide, about 1 to 30 mol. percent of copper oxidecalculated as CuO, and the remainder nickel oxide is present, shapingsaid mixture into a body, and sintering said body at a temperaturebetween about 1100 C. and 1300 C. in an oxidizing atmosphere.

4. A method of manufacturing a sintered electrical resistor having anegative temperature coefficient comprising forming a mixture ofcompound which form the oxides of Ni, Li and Cu upon heating in amountssuch that from about 0.1 to 10 mol. percent of lithium oxide, about 1 to30 mol. percent of copper oxide calculated as CuO, and the remaindernickel oxide is present, shaping said mixture into a body, and sinteringsaid body at a temperature between about 1100 C. and 1300" C. in air.

References Cited in the file of this patent UNITED STATES PATENTS2,326,580 Trenkle Aug. 10, 1943 FOREIGN PATENTS 545,679 Great BritainJune 8, 1942 OTHER REFERENCES Philips: Res. Rep., vol. 5, pages 173-487,1950.

3. A METHOD OF MANUFACTURING A SINTERED ELECTRICAL RESISTOR HAVING ANEGATIVE TEMPERATURE COEFFICIENT COMPRISING FORMING A MIXTURE OF THEOXIDES OF NI, LI AND CU IN AMOUNTS SUCH THAT FROM ABOUT 0.1 TO 10 MOL.PERCENT OF LITHIUM OXIDE, ABOUT 1 TO 30 MOL. PERCENT OF COPPER OXIDECALCULATED AS CUO, AND THE REMAINDER NICKEL OXIDE IS PRESENT, SHAPINGSAID MIXTURE INTO A BODY, AND SINTERING SAID BODY AT A TEMPERATUREBWTWEEN ABOUT 1100* C. AND 1300*C. IN AN OXIDIZING ATMOSPHERE.